BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to an article and method comprising a catalyst composition useful for the treatment of gases to reduce pollutants contained therein. More specifically, the present invention is concerned with catalysts of the type generally referred to as “close coupled catalysts” which are designed to reduce pollutants in engine exhaust emissions during engine cold start conditions.
2. Description of the Related Art
California Low Emission Vehicle standards require significantly higher emissions reduction, especially for hydrocarbon and nitrogen oxides. For a typical vehicle, a large portion (up to 80%) of the hydrocarbon emissions occurs during the first phase of the Federal Test Procedure (“FTP”). A variety of technologies are under development to reduce cold start hydrocarbon emissions, including close coupled catalysts as disclosed in Ball, D. J., “Distribution of Warm-Up and Underfloor Catalyst Volumes”, SAE 922338, 1992; electrically heated catalysts as disclosed in Piotrowski, G. K., “Evaluation of a Resistively Heated Metal Monolith Catalytic Converter on a Gasoline-Fueled Vehicle”, EPA/AA/CTAAB/88-12, 1988 and Hurley, R. G. “Evaluation of Metallic and Electrically Heated Metallic Catalysts on a Gasoline Fueled Vehicle”, SAE 900504, 1990; hydrocarbon absorbers as disclosed in Heimrich, M. J., Smith, L. R., and Kitowski, J., “Cold Start Hydrocarbon Collection for Advanced Exhaust Emission Control, SAE 920847, 1992 and Hochmuth, J. K., Burk, P. L., Telentino, C., and Mignano, M. J., “Hydrocarbon Traps for Controlling Cold Start Emissions”, SAE 930739, 1993; by-pass catalysts as disclosed in Fraidl, G. K., Quissrk, F. and Winklhofer, E., “Improvement of LEV/ULEV Potential of Fuel Efficient High Performance Engines,” SAE 920416, 1992; and burners as disclosed in Ma, T., Collings, N. and Hands, T., “Exhaust Gas Ignition (EGI)—A New Concept for Rapid Light-off of Automotive Exhaust Catalyst, SAE 920400, 1992. It has been reported that close coupled catalysts, especially Pd-containing catalysts, are very effective at reducing HC emission during cold start of the FTP cycle as disclosed in Ball, D. J., “Distribution of Warm-Up and Underfloor Catalyst Volumes”, SAE 922338, 1992; Summers, J. C., Skowron, J. F., and Miller, M. J., “Use of Light-Off Catalysts to Meet the California LEV/ULEV Standards”, SAE 930386, 1993 and Ball, D. J., “A Warm-up and Underfloor Converter Parametric Study”, SAE 932765, 1993. Recently, Ford has reported a successful application of Pd-only catalyst for meeting stringent emission standards as disclosed in Dettling, J., Hu, Z, Lui, Y., Smaling, R., Wan, C and Punke, A., “SMART Pd TWC Technology to Meet Stringent Standards”, Presented at CAPoC3 Third International Congress on Catalyst and Automobile Pollution Control, Apr. 20-22, 1994, Brussels.
The principal function of a close coupled catalyst, also referred to as “precat” and “warm-up” catalysts, is to reduce hydrocarbon emissions during cold start. Cold start is the period immediately after starting the engine from ambient conditions. The cold start period depends on the ambient temperature, the type of engine, the engine control system and engine operation. Typically, the cold start period is within the first two minutes after the start of an engine at ambient temperature, FTP Test 1975 characterize cold start as the first bag of the FTP driving cycle which lasts for the first 505 seconds after starting an engine from ambient temperature, typically at 26° C. This is accomplished by locating at least part of the total exhaust system catalyst closer to the engine than traditional “underfloor catalyst”. The underfloor catalyst are typically located beneath the floor of the vehicle. The close coupled catalyst is located in the engine compartment, i.e., beneath the hood and adjacent to the exhaust manifold. There are two possible strategies for implementing a close coupled catalyst. The close coupled catalyst can occupy the entire catalyst volume or be a small volume catalyst used in conjunction with an underfloor catalyst. The design option depends on the engine configuration, size and space available.
Catalysts at the close coupled position are also exposed to high temperature exhaust gas immediately exiting the engine after the engine has warmed up. As a consequence, the close coupled catalyst must have high temperature stability to be durable enough for meeting stringent emission standards as disclosed in Bhasin, M. et al, “Novel Catalyst for Treating Exhaust Gases from Internal Combustion and Stationary Source Engines”, SAE 93054, 1993. In the present day vehicle control strategies, overfueling or fuel enrichment is used to cool the engine exhaust prior to the catalyst during high load operation or high exhaust temperature conditions. This strategy results in increased hydrocarbon emissions and may be eliminated in future regulations as disclosed in “Acceleration Enrichment May Be Large Source of Pollution”, WARD'S Engine and Vehicle Technology Update, Dec. 1, 1993, p.4. This could result in 50 to 100° higher exposure temperatures for the catalyst. Thus, the close coupled catalyst could be exposed to temperatures as high as 1050° C. Additionally, high speed Autobahn driving conditions can expose the close coupled catalyst to such high temperatures.
A typical motor vehicle catalyst is an underfloor three-way conversion catalysts (“TWC”) which catalyzes the oxidation by oxygen in the exhaust gas of the unburned hydrocarbons and carbon monoxide and the reduction of nitrogen oxides to nitrogen. TWC catalysts which exhibit good activity and long life comprise one or more platinum group metals (e.g., platinum or palladium, rhodium, ruthenium and iridium) located upon a high surface area, refractory oxide support, e.g., a high surface area alumina coating. The support is carried on a suitable carrier or substrate such as a monolithic carrier comprising a refractory ceramic or metal honeycomb structure, or refractory particles such as spheres or short, extruded segments of a suitable refractory material.
U.S. Pat. No. 4,134,860 relates to the manufacture of catalyst structures. The catalyst composition can contain platinum group metals, base metals, rare earth metals and refractory, such as alumina support. The composition can be deposited on a relatively inert carrier such as a honeycomb.
The high surface area alumina support materials, also referred to as “gamma alumina” or “activated alumina”, typically exhibit a BET surface area in excess of 60 square meters per gram (“m2/g”), often up to about 200 m2/g or more. Such activated alumina is usually a mixture of the gamma and delta phases of alumina, but may also contain substantial amounts of eta, kappa and theta alumina phases. It is disclosed to utilize refractory metal oxides other than activated alumina as a support for at least some of the catalytic components in a given catalyst. For example, bulk ceria, zirconia, alpha alumina and other materials are known for such use. Although many of these materials suffer from the disadvantage of having a considerably lower BET surface area than activated alumina, that disadvantage tends to be offset by a greater durability of the resulting catalyst.
In a moving vehicle, exhaust gas temperatures can reach 1000° C., and such elevated temperatures cause the activated alumina (or other) support material to undergo thermal degradation caused by a phase transition with accompanying volume shrinkage, especially in the presence of steam, whereby the catalytic metal becomes occluded in the shrunken support medium with a loss of exposed catalyst surface area and a corresponding decrease in catalytic activity. It is a known expedient in the art to stabilize alumina supports against such thermal degradation by the use of materials such as zirconia, titania, alkaline earth metal oxides such as baria, calcia or strontia or rare earth metal oxides, such as ceria, lanthana and mixtures of two or more rare earth metal oxides. For example, see C. D. Keith et al U.S. Pat. No. 4,171,288.
Bulk cerium oxide (ceria) is disclosed to provide an excellent refractory oxide support for platinum group metals other than rhodium, and enables the attainment of highly dispersed, small crystallites of platinum on the ceria particles, and that the bulk ceria may be stabilized by impregnation with a solution of an aluminum compound, followed by calcination. U.S. Pat. No. 4,714,694 of C. Z. Wan et al, discloses aluminum-stabilized bulk ceria, optionally combined with an activated alumina, to serve as a refractory oxide support for platinum group metal components impregnated thereon. The use of bulk ceria as a catalyst support for platinum group metal catalysts other than rhodium, is also disclosed in U.S. Pat. No. 4,727,052 of C. Z. Wan et al and in U.S. Pat. No. 4,708,946 of Ohata et al.
U.S. Pat. No. 4,923,842 discloses a catalytic composition for treating exhaust gases comprising a first support having dispersed thereon at least one oxygen storage component and at least one noble metal component, and having dispersed immediately thereon an overlayer comprising lanthanum oxide and optionally a second support. The layer of catalyst is separate from the lanthanum oxide. The nobel metal can include platinum, palladium, rhodium, ruthenium and iridium. The oxygen storage component can include the oxide of a metal from the group consisting of iron, nickel, cobalt and the rare earths. Illustrative of these are cerium, lanthanum, neodymium, praseodymium, etc. Oxides of cerium and praseodymium are particularly useful as oxygen storage components.
U.S. Pat. No. 4,808,564 discloses a catalyst for the purification of exhaust gases having improved durability which comprises a support substrate, a catalyst carrier layer formed on the support substrate and catalyst ingredients carried on the catalyst carrier layer. The catalyst carrier layer comprises oxides of lanthanum and cerium in which the molar fraction of lanthanum atoms to total rare earth atoms is 0.05 to 0.20 and the ratio of the number of the total rare earth atoms to the number of aluminum atoms is 0.05 to 0.25.
U.S. Pat. No. 4,438,219 discloses an alumina supported catalyst for use on a substrate. The catalyst is stable at high temperatures. The stabilizing material is disclosed to be one of several compounds including those derived from barium, silicon, rare earth metals, alkali and alkaline earth metals, boron, thorium, hafnium and zirconium. Of the stabilizing materials barium oxide, silicon dioxide and rare earth oxides which include lanthanum, cerium, praseodymium, neodymium, and others are indicated to be preferred. It is disclosed that contacting them with a calcined alumina film permits the calcined alumina film to retain a high surface area at higher temperatures.
U.S. Pat. Nos. 4,476,246, 4,591,578 and 4,591,580 disclose three-way catalyst compositions comprising alumina, ceria, an alkali metal oxide promoter and noble metals. U.S. Pat. No. 4,591,518 discloses a catalyst comprising an alumina support with components deposited thereon consisting essentially of a lanthana component, ceria, an alkali metal oxide and a platinum group metal. U.S. Pat. No. 4,591,580 discloses an alumina supported platinum group metal catalyst. The support is sequentially modified to include support stabilization by lanthana or lanthana rich rare earth oxides, double promotion by ceria and alkali metal oxides and optionally nickel oxide.
Palladium containing catalyst compositions e.g. U.S. Pat. No. 4,624,940 have been found useful for high temperature applications. The combination of lanthanum and barium is found to provide a superior hydrothermal stabilization of alumina which supports the catalytic component, palladium.
U.S. Pat. No. 4,780,447 discloses a catalyst which is capable of controlling HC, CO and NOx as well as H2S in emissions from the tailpipe of catalytic converter equipped automobiles. The use of the oxides of nickel and/or iron is disclosed as a H2S gettering of compound.
U.S. Pat. No. 4,965,243 discloses a method to improve thermal stability of a TWC catalyst containing precious metals by incorporating a barium compound and a zirconium compound together with ceria and alumina. This is stated to form a catalytic moiety to enhance stability of the alumina washcoat upon exposure to high temperature.
J01210032 (and AU-615721) discloses a catalytic composition comprising palladium, rhodium, active alumina, a cerium compound, a strontium compound and a zirconium compound. These patents suggests the utility of alkaline earth metals in combination with ceria, zirconia to form a thermally stable alumina supported palladium containing washcoat.
U.S. Pat. Nos. 4,624,940 and 5,057,483 refer to ceria-zirconia containing particles. It is found that ceria can be dispersed homogeneously throughout the zirconia matrix up to 30 weight percent of the total weight of the ceria-zirconia composite to form a solid solution. A co-formed (e.g. co-precipitated) ceria oxide-zirconia particulate composite can enhance the ceria utility in particles containing ceria-zirconia mixture. The ceria provides the zirconia stabilization and also acts as an oxygen storage component. The '483 patent discloses that neodymium and/or yttrium can be added to the ceria-zirconia composite to modify the resultant oxide properties as desired.
U.S. Pat. No. 4,504,598 discloses a process for producing a high temperature resistant TWC catalyst. The process includes forming an aqueous slurry of particles of gamma or activated alumina and impregnating the alumina with soluble salts of selected metals including cerium, zirconium, at least one of iron and nickel and at least one of platinum, palladium and rhodium and, optionally, at least one of neodymium, lanthanum, and praseodymium. The impregnated alumina is calcined at 600° C. and then dispersed in water to prepare a slurry which is coated on a honeycomb carrier and dried to obtain a finished catalyst.
Japanese Kokai 71538/87 discloses a catalyst layer supported on a catalyst carrier and containing one catalyst component selected from the group consisting of platinum, palladium and rhodium. An alumina coat layer is provided on the catalyst layer. The coat layer contains one oxide selected from the group consisting of cerium oxide, nickel oxide, molybdenum oxide, iron oxide and at least one oxide of lanthanum and neodymium (1-10% by wt.).
U.S. Pat. Nos. 3,956,188 and 4,021,185 disclose a catalyst composition having (a) a catalytically active, calcined composite of alumina, a rare earth metal oxide and a metal oxide selected from the group consisting of an oxide of chromium, tungsten, a group IVB metal and mixtures thereof and (b) a catalytically effective amount of a platinum group metal added thereto after calcination of said composite. The rare earth metals include cerium, lanthanum and neodymium.
Japanese Patent J-63-077544-A discloses a layered automotive catalyst having a first layer comprising palladium dispersed on a support comprising alumina, lanthana and other rare earth oxides and a second coat comprising rhodium dispersed on a support comprising alumina, zirconia, lanthana and rare earth oxides.
U.S. Pat. No. 4,587,231 discloses a method of producing a monolithic three-way catalyst for the purification of exhaust gases. First, a mixed oxide coating is provided to a monolithic carrier by treating the carrier with a coating slip in which an active alumina powder containing cerium oxide is dispersed together with a ceria powder and then baking the treated carrier. Next platinum, rhodium and/or palladium are deposited on the oxide coating by a thermal decomposition. Optionally, a zirconia powder may be added to the coating slip.
U.S. Pat. No. 5,057,483 discloses a catalyst composition suitable for three-way conversion of internal combustion engine, e.g., automobile gasoline engine, exhaust gases includes a catalytic material disposed in two discrete coats on a carrier. The first coat includes a stabilized alumina support on which a first platinum catalytic component is dispersed and bulk ceria, and may also include bulk iron oxide, a metal oxide (such as bulk nickel oxide) which is effective for the suppression of hydrogen sulfide emissions, and one or both of baria and zirconia dispersed throughout the first coat as a thermal stabilizer. The second coat, which may comprise a top coat overlying the first coat, contains a co-formed (e.g., co-precipitated) rare earth oxide-zirconia support on which a first rhodium catalytic component is dispersed, and a second activated alumina support having a second platinum catalytic component dispersed thereon. The second coat may also include a second rhodium catalytic component, and optionally, a third platinum catalytic component, dispersed as an activated alumina support.
It is a continuing goal to develop a close coupled catalyst system which is inexpensive and stable. The system should have the ability to oxidize hydrocarbons at low temperatures.
SUMMARY OF THE INVENTION
The present invention relates to a stable close-coupled catalyst, an article comprising such a close-coupled catalyst and a related method of operation.
The close-coupled catalyst of the present invention has been designed to reduce hydrocarbon emissions from gasoline engines during cold starts. More particularly, the close-coupled catalyst is designed to reduce pollutants in automotive engine exhaust gas streams at temperatures as low as 350° C., preferably as low as 300° C. and more preferably as low as 200° C. The close-coupled catalyst of the present invention comprises a close-coupled catalyst composition which catalyzes low temperature reactions. This is indicated by the light-off temperature. The light-off temperature for a specific component is the temperature at which 50% of that component reacts.
The close-coupled catalyst is placed close to an engine to enable it to reach reaction temperatures as soon as possible. However, during steady state operation of the engine, the proximity of the close-coupled catalyst to the engine, typically less than one foot, more typically less than six inches and commonly attached directly to the outlet of the exhaust manifold exposes the close-coupled catalyst composition to exhaust gases at very high temperatures of up to 1100° C. The close-coupled catalyst in the catalyst bed is heated to high temperature by heat from both the hot exhaust gas and by heat generated by the combustion of hydrocarbons and carbon monoxide present in the exhaust gas. In addition to being very reactive at low temperatures, the close-coupled catalyst composition should be stable at high temperatures during the operating life of the engine.
As indicated in the Background of the Invention, gasoline engines typically release exhaust gas pollutants which include hydrocarbons, carbon monoxide and nitric oxides. Typical catalytic converters are located “under the floor” of the automobile. Such catalytic converters comprise catalyst compositions which act as “three-way catalysts” (TWC). The TWC catalysts oxidize carbon monoxide and hydrocarbons and reduce nitric oxides. The carbon monoxide is oxidized to carbon dioxide and the hydrocarbons are oxidized to water and carbon dioxide. The nitric oxide is typically reduced to nitrogen gas.
The close-coupled catalyst present invention accomplishes the oxidation of carbon monoxide and hydrocarbons and reduction of nitrogen oxides at “cold start” conditions reviewed in the Background. Such conditions are as low as 350° C., preferably 300° C. and more preferably as low as 200° C. At the same time, the close-coupled catalyst composition is thermally stable upon exposure to temperature up to 1100° C. and higher during the operating life of the engine. This has been accomplished by increasing thermal stability of the catalyst washcoat, and by controlling the reaction of carbon monoxide in the close-coupled catalyst bed and therefore reducing temperature rise related to carbon monoxide combustion in the catalyst bed. At the same time, the close-coupled catalyst compositions provides a relatively high hydrocarbon conversion. A catalyst downstream of the close-coupled catalyst can be an underfloor catalyst or a downstream catalyst. When the underfloor catalyst is heated to a high enough temperature to reduce the pollutants, the reduced conversion of carbon monoxide in the close-coupled catalyst results in a cooler close-coupled catalyst and enables the downstream catalyst typically the underfloor three-way catalyst to burn the carbon monoxide and run more effectively at a higher temperature.
The close-coupled catalyst composition of the present invention comprises components of the type used in a TWC catalyst composition except that there is substantially no oxygen storage components. The removal of the oxygen storage components from the close-coupled catalyst composition of the present invention results in controlled bypass of carbon monoxide. For the purposes of the present invention, components which have oxygen storage and release capabilities include cerium oxide and praseodymium oxide. Equivalent amount of other rare earths having less significant oxygen storage capability are not considered to be components which have substantial oxygen storage and release capability. Additionally, platinum group metal components are not considered to be oxygen storage components. In particular, the close-coupled catalyst composition can be a three-way catalyst composition having substantially no ceria. Minor amounts of ceria or praseodymium may be present as impurities or trace amounts. Oxygen storage component such as cerium oxide store oxygen and release it during operating conditions providing additional oxygen to enable the oxidation of hydrocarbons and carbon monoxides to proceed more efficiently. However, this function has been found to result in excess oxidation and overheating of the close-coupled catalyst.
The present composition includes a palladium component, preferably at relatively high concentration. Accordingly, during cold start operation, a relatively high amount of hydrocarbons are oxidized, and a significant amount of carbon monoxide, although not all of the carbon monoxide is oxidized. Additionally, a significant amount of nitrogen oxides are reduced. In addition, the absence of oxygen storage component, particularly cerium compounds in the close-coupled catalyst limits the amount of carbon monoxide oxidation in the close-coupled catalyst even when the engine exhaust gases are hot and the downstream (underfloor) catalyst has reached operating temperatures. The carbon monoxide which does not react in the close-coupled catalyst passes to the downstream catalyst where it is catalytically oxidized, and such oxidation increases the temperature of the downstream catalyst resulting in a more effective operation. Accordingly, the close-coupled catalyst of the present invention is sufficiently effective to eliminate a significant amount of pollutants at low temperatures while at the same time being stable over long periods of engine operation while providing a sufficient amount of carbon monoxide to the downstream catalyst to permit it to operate effectively.
The present invention includes an article comprising a gasoline engine having an exhaust outlet, typically connected in communication to the inlet of an exhaust manifold. The close-coupled catalyst is in communication with the exhaust outlet and is typically connected in communication with the exhaust manifold outlet. The close-coupled catalyst can be connected directly to the gasoline engine outlet or exhaust manifold outlet. Alternatively, it can be connected by a short exhaust pipe, typically up to about one foot long to the exhaust outlet or exhaust manifold outlet of the gasoline engine. The close-coupled catalyst has an outlet which is connected in communication with the inlet of the downstream preferably underfloor catalytic converter. Exhaust pipes can be connected from the outlet of the close-coupled catalyst outlet and the inlet of the underfloor catalytic converter inlet. The underfloor catalytic converter has an outlet which can be connected to outlet exhaust pipes through which the exhaust gas passes from the vehicle into the atmosphere. The close-coupled catalyst comprises a close-coupled catalyst composition. The underfloor catalyst preferably comprises a three-way catalyst composition containing ceria.
The close-coupled catalyst composition of the present invention is substantially free of oxygen storage components such as ceria and praseodymia. The catalyst composition comprises a support which preferably comprises at least one compound selected from the group consisting of silica, alumina, titania and a first zirconia compound hereinafter referred to as a first zirconia compound. The composition further comprises a palladium component, preferably in an amount sufficient to oxidize carbon monoxide and hydrocarbons and reduce nitric oxides to have respective light-off temperatures at 50% conversion which are relatively low and preferably in the range of from 200 to 350° C. for the oxidation of hydrocarbons. The composition optionally comprises at least one alkaline metal oxide selected from the group consisting of strontium oxide, calcium oxide and barium oxide with strontium oxide most preferred. The composition can optionally also comprise other precious metal or platinum group metal components, preferably including at least one metal selected from the group consisting of platinum, rhodium, ruthenium and iridium components. Where additional platinum group metals are included, if platinum is used, it is used in an amount of less than 60 grams per cubic foot. Other platinum group metals are used in amounts of up to about 20 grams per cubic foot. The composition optionally also can include a second zirconium oxide compound as a stabilizer and optionally at lease one rare earth oxide selected from the group consisting of neodymium oxide and lanthanum oxide.
The close-coupled catalyst preferably is in the form of a carrier supported catalyst where the carrier comprises a honeycomb type carrier. A preferred honeycomb type carrier comprises a composition having at least about 50 grams per cubic foot of palladium component, from 0.5 to 3.5 g/in3 of activated alumina, and from 0.05 to 0.5 g/in3 of at least one alkaline earth metal component, most preferably, strontium oxide. Where lanthanum and/or neodymium oxide are present, they are present in amounts up to 0.6 g/in3.
The present invention comprises a method of operating a gasoline engine having an exhaust which comprises pollutants including carbon monoxide, hydrocarbons and optionally nitrogen oxide. The exhaust gas stream is passed from the engine outlet to the inlet of a close-coupled catalyst of the type described above. The gases contact with the close-coupled catalyst and reacts. The close-coupled catalyst has substantially no oxygen storage components, particularly ceria and praseodymia components. The exhaust gas can then pass to a downstream three-way catalyst which preferably comprises an oxygen storage component such as ceria.
In an optional embodiment, the three-way catalyst is included as part of the close-coupled catalytic article on a carrier which is within the close-coupled catalyst canister downstream from the close-coupled catalyst carrier.